1. Field of the Invention
The present invention relates to a process for a separation of naphthoquinone and phthalic acid. More particularly, it relates to a process for a separation of naphthoquinone at high purity from an aqueous slurry containing 1,4-naphthoquinone (hereinafter 1,4-naphthoquinone is referred to as naphthoquinone) obtained by washing, with water, a reaction mixture gas obtained by a catalytic vapor phase oxidation of naphthalene.
2. Description of Prior Arts
It has been proposed to separate naphthoquinone and phthalic acid from an aqueous slurry obtained by washing, with water, naphthoquinone and phthalic anhydride by contacting an aqueous medium, with a reaction mixture gas obtained by a catalytic vapor phase oxidation of naphthalene, wherein the naphthoquinone is extracted from the aqueous slurry with an aromatic hydrocarbon solvent such as xylene to separate a naphthoquinone solution from an aqueous solution of phthalic acid (See Japanese Examined Patent Publication No. 20026/1978.).
It has been also proposed in Japanese Examined Patent Publication No. 9209/1978 to separate an insoluble naphthoquinone crystal by adding a base to an aqueous medium obtained by washing, with water, the reaction mixture gas obtained by the catalytic vapor phase oxidation, to dissolve phthalic acid as a mono-salt of phthalic acid. Thus, the purity of naphthoquinone obtained by these known processes is too low. In the former process, polycondensed quinones and resinous materials contained in the aqueous slurry of naphthoquinone and phthalic acid obtained by washing, with water, the reaction mixture gas obtained by the catalytic vapor phase oxidation of naphthalene, are dissolved into the aromatic hydrocarbon used for the extraction, whereby the solution of naphthoquinone contains the impurities.
In the latter process, as described in the specification especially the examples, the impurities of the polycondensed products and resinous materials which are insoluble to the aqueous phase is included at a ratio of about 5%, in naphthoquinone crystal and naphthoquinone having high purity cannot be obtained.
Naphthoquinone is a remarkably important industrial compound as an intermediate of dyes and agricultural chemicals. It has been required to produce naphthoquinone having high purity, however, it has not been succeeded to produce naphthoquinone having high purity by an industrially advantageous process from the reaction mixture gas obtained by the catalytic oxidation of naphthoquinone.
The inventors have studied to obtain naphthoquinone having high purity from the reaction mixture gas formed by the catalytic vapor phase oxidation of naphthalene. As a result, it has found that the impurities such as polycondensed products and resinous materials are not substantially dissolved in the saturated hydrocarbon solvent phase, but only naphthoquinone is selectively extracted into the saturated hydrocarbon solvent phase to easily separate the impurities such as the polycondensed products and the resinous materials as the insoluble materials by using the saturated hydrocarbon solvent in the extraction of naphthoquinone from the aqueous slurry containing naphthoquinone and phthalic acid, the pH of which is adjusted to the specific range so as to prevent the polycondensation of naphthoquinone.